1. Field of the Invention
This invention relates generally to a hydrostatic drive system including a pump and a hydraulic motor located in an open circuit and more particularly to a hydrostatic drive system having a control valve which determines the quantity of hydraulic pressure medium on the inflow side of the hydraulic motor and a brake valve which is responsive to the quantity of hydraulic pressure medium on the outflow side of the hydraulic motor wherein the control valve and the brake valve are acted upon by the same pressure.
Brake valves are included in a hydrostatic drive system to prevent cavitation at the hydraulic motor. If brake valves are not provided in the system, cavitation will occur when energy is supplied to the hydraulic motor from the driven side in which case the motor operates as a pump. In this situation the pressure ratios in the two feed lines to the hydraulic motor are reversed which results in an insufficient supply of hydraulic medium to the hydraulic motor. Moreover, there is the risk that the hydraulic motor will exceed its rated speed. This situation can occur, for example, when a hydrostatic drive system is braked and/or during downhill travel.
Actuation of the brake valves depends on the pressure in the feed line to the inflow side of the hydraulic motor (the pressure on the inflow side of the hydraulic motor acts directly on a control surface of the brake valve). Each brake valve on the outflow side of the hydraulic motor is preloaded in the direction toward the closed position by a spring and is maintained in the open position by the pressure of the hydraulic medium on the inflow side of the hydraulic motor. As soon as the pressure of the hydraulic medium on the inflow side of the hydraulic motor falls below a certain predetermined value, the brake valve closes (as a rule this occurs continuously, i.e., with throttling in intermediate positions) and reduces the cross-section of the feed line on the outflow side of the hydraulic motor to produce a throttle effect.
2. Description of the Prior Art
Conventional hydrostatic drive systems include two brake valves, which may be located on a common axis, and one or two control valves on an axis spaced therefrom. Therefore, at least two parallel bores always have to be accommodated when the valves are located in housings.
German Patent No. 2,220,571 discloses integrating the brake valve in the control valve and the valve sub-assembly which is formed in this manner can be located on a common axis to eliminate the above-mentioned disadvantage. The valve sub-assembly is actuated by an actuating pressure. If the pressure on the inflow side of the hydraulic motor falls below the actuating pressure threshold in the braking phase, the actuating pressure decreases in proportion to the inflow pressure. However, this does not prohibit the occurrence of instabilities during certain operating phases. Particularly in the braking phase, i.e. when the pressure on the inflow side of the hydraulic motor falls below a certain value and as a result the valve sub-assembly containing the brake valve moves in the direction of closure, the outflow cross-section is reduced to produce a throttle effect, and the inflow cross-section is reduced at the same time in this known design. This causes the inflowing hydraulic pressure medium to be throttled back and the pressure on the inflow side falls even more sharply, because the valve sub-assembly moves further in the direction of closure. This interactive coupling effect leads to oscillations in the control system and braking becomes questionable.